As is known in the art, alumina trihydrate ("ATH") contains about 34.6% by weight chemically bonded water of hydration which acts as a heat sink, thus retarding ignition and combustion of materials in which it is incorporated. During combustion of such materials, the water tends to evolve as steam, absorbing energy for the conversion of the water phase, inhibiting flame spread, limiting the accessibility of oxygen, and suppressing smoke development.
ATH is therefore commonly used as a filler and flame retardant in synthetic "solid surface" materials such as are used for kitchen countertops and other architectural and furniture applications. The alumina trihydrate may be incorporated in resin matrices of various composition, such as disclosed in Buser's U.S. Pat. Nos. 4,085,246 and 4,159,301, or it may be encased in resin particles which are employed as part of granite or other synthetic mineral appearing patterns. In the latter case, the encasement is accomplished by first preparing a mixture of ATH, pigment or dye and resin such as polyester, then curing or hardening the resin incorporating the ATH, and then crushing or grinding the hardened "ingot" to obtain colored particles which can then be used to impart decorative patterns and flame retardancy in a further mixture with hardenable resin such as polyester or polymethylmethacrylate. See Ross and Risley U.S. Pat. No. 4,961,995. The ATH is not colored, however, (cf Duggins U.S. Pat. No. 3,847,865) and typically is quite small so that the particles are not readily discernible in the desired mineral pattern. The speckled appearance of a granite pattern, for example, is usually imparted by relatively large (that is, visually discernible) particles of colored resin, which may or may not have ATH in them.
The amount of ATH which can be incorporated in a mineral-appearing material is limited in the above procedures by the necessity of coloring the particles. Also, ATH particles large enough to be visible in a mineral pattern have generally not been employed because when the surface (such as a kitchen countertop) is sanded or polished, the ATH particles near the surface will also be sanded or polished, and the appearance of their uncolored interiors thereby exposed is generally not compatible with the desired visual effect. A similar phenomenon known as "white-capping" is observable from scratching or abrasion of the finished synthetic material, wherein the colored resin encasement is removed and exposes the relatively white ATH. See Gavin et al U.S. Pat. No. 4,413,089, referring to this phenomenon as "scratch white" in column 2, lines 7-12.
So far as I am aware, colored ATH particles suitable for use in making mineral-appearing resin materials have not been previously made. The above-mentioned Gavin patent, for example, uses iron oxide to impart color to a polymethylmethacrylate article filled with ATH. Colorants, dyes, pigments, and lakes, organic and inorganic, soluble and insoluble, are discussed extensively in Duggins U.S. Pat. No. 3,488,246 without hinting that a water-soluble dye could be incorporated into ATH by a rehydration procedure. ATH which has been incorporated with dye has not been used to make simulated marble, granite or other mineral product, so far as I am aware.
U.S. Pat. No. 2,378,155 to Newsome and Derr describes a process of making an adsorbent alumina from a gelatinous aluminum hydroxide and goes on to say that the adsorbent alumina, which is calcined, can be impregnated with "various substances." See page 2, column 1, lines 3-16. The statement is made, however, without reference to coloration of hydrated alumina for the purpose of making synthetic minerals or any other purpose, or to the particle size, or to the possibility of a beginning material which is ATH of the type widely used commercially today.
A paper authored by P. V. Bonsignora and J. H. Manhart of Aluminum Company of America and presented at the 29th conference of the Society for the Plastics Industry in 1974 demonstrates that through thermogravimetric analysis it is possible to confirm that water evolution from decomposing alumina trihydrate becomes significant at about 230.degree. C. and is greater than 80% complete at 300.degree. C. At about 300.degree. C. (for about eighty minutes) approximately 80% of the water of hydration is driven off; the last 20% is more difficult to remove. If more than an additional 5% is removed (i.e. the hydration is reduced from 20% to 15%), the crystalline structure of the alumina may become permanently altered so as to prevent rehydration in the manner as to reconstitute alumina trihydrate.